Only Contents help to optimize a blog? or backlinks needed too?

Both are important content and backlink and you have to find normal competitive niche.

I don't bother anymore with Web 2.0s. I start building quality PBN links, niche edits and guest posts from the start. The only thing that changes later on is the speed at which I build links.

As for the content part, I feel like people don't research their content nearly as much as they should. Check all the keywords and make sure to include answers to all the common questions you can find on the Web. If you're hiring a new writer make sure to give them a clear article structure or template so they cover the important stuff.

If u asked what u should do beside of publishing content routinely. The next step is by making PBNs, if you have enough time and money, hire them in the trusted sellers. If u know how to change aged domain to be powerful PBN, do it!.
Do a content placement, means put your link in the sites with high DA PA with little bit spam score. Use tools like Ahref and Moz might be helpful.

OP, you maybe able to to get away with no backlinks IF you do the following

- focus on low competition keywords AND
- go heavy on outreach

Content is the key ingredient..but it is not the only factor..backlinks are also needed so better start building them asap...just make sure they look natural...